LOS ANGELES - U.S. immigration agents descended on dozens of 7-Eleven stores before dawn Wednesday to open employment audits and interview workers in what officials described as the largest operation against an employer under Donald Trump's presidency.

Agents targeted about 100 stores nationwide, broadening an investigation that began with a 4-year-old case against a franchisee on New York's Long Island. The audits could lead to criminal charges or fines over the stores' hiring practices.

The action appears to open a new front in Trump's sharp expansion of immigration enforcement, which has already brought a 40 percent increase in deportation arrests and plans to spend billions of dollars on a border wall with Mexico. Hardliners have been pressing for a tougher stance on employers.

Derek Benner, a top official at U.S. Immigration and Customs Enforcement, told The Associated Press that Wednesday's operation was 'the first of many' and 'a harbinger of what's to come' for employers. He said there would be more employment audits and investigations, though there is no numerical goal.

'This is what we're gearing up for this year and what you're going to see more and more of is these large-scale compliance inspections, just for starters. From there, we will look at whether these cases warrant an administrative posture or criminal investigation,' said Benner, acting head of ICE's Homeland Security Investigations, which oversees cases against employers.

'It's not going to be limited to large companies or any particular industry, big medium and small,' he said. 'It's going to be inclusive of everything that we see out there.'

7-Eleven Stores Inc., based in Irving, Texas, with more than 8,600 stores in the U.S., didn't immediately respond to a message seeking comment.

Though agents arrested 21 people suspected of being in the country illegally during Wednesday's sweep, the action was aimed squarely at management.

Illegal hiring is rarely prosecuted, partly because investigations are time-consuming and convictions are difficult to achieve because employers can claim they were duped by fraudulent documents or intermediaries. Administrative fines are discounted by some as a business cost.

George W. Bush's administration aggressively pursued criminal investigations against employers in its final years with dramatic pre-dawn shows of force and large numbers of worker arrests. In 2008, agents arrived by helicopter at the Agriprocessors meatpacking plant in Postville, Iowa, and detained nearly 400 workers. Last month, Trump commuted the 27-year prison sentence of Sholom Rubashkin, former chief executive of what was the nation's largest kosher meatpacking operation.

Barack Obama's administration more than doubled employer audits to more than 3,100 a year in 2013, shunning Bush's flashier approach. John Sandweg, an acting ICE director under Obama, said significant fines instilled fear in employers and draining resources from other enforcement priorities.

Trump is pursuing 'its own kind of unique strategy' tied to its broader emphasis on fighting illegal immigration, including enforcement on the border, Benner said. Some workers may get arrested in the operations but authorities are targeting employers because they are job magnets for people to come to the country illegally.

'We need to make sure that employers are on notice that we are going to come out and ensure that they're being compliant,' Benner said 'For those that don't, we're going to take some very aggressive steps in terms of criminal investigations to make sure that we address them and hold them accountable.'

Wednesday's operation resulted from a 2013 investigation that resulted in charges against nine 7-Eleven franchisees and managers in New York and Virginia. Eight have pleaded guilty and were ordered to pay more than $2.6 million in back wages, and the ninth was arrested in November.

In the 2013 investigations, managers used more than 25 stolen identities to employ at least 115 people in the country illegally, knowing they could pay below minimum wage, according to court documents. The documents say 7-Eleven corporate office does automated payroll, requiring franchisees provide employee names and Social Security numbers to pay workers through direct deposit or check.

The 7-Eleven stores served on Wednesday will be required to produce documents showing they required work authorization, which Benner said will become more common. Audits may lead to criminal charges or administrative penalties.

In Los Angeles' Koreatown, seven agents arriving in three unmarked cars closed a store for 20 minutes to explain the audit to the only employee there, a clerk with a valid green card. Agents, wearing blue jackets marked ICE, told arriving customers that the store was closed briefly for a federal inspection. A driver delivering cases of beer was told to wait in the parking lot.

The manager was in Bangladesh and the owner, reached by phone, told the clerk to accept whatever documents were served. Agents said they would return Tuesday for employment records they requested.

Neither 7-Eleven nor was its parent company, Seven and i Holding Co. based in Tokyo, was charged in that case.

'Just as the IRS performs audits of people all the time of their tax returns, the same purpose here is to ensure a culture of compliance in this area,' he said.